In agriculturally advanced countries the days of farmers planting crops, spreading fertilizer, and then hoping for the best until harvest time, are long gone. Nowadays farming is akin to managing a precision biochemical factory and plants are carefully monitored to maximize their growth. Fertilizer application is done according to specific algorithms that lead to high crop yield.
Precision fertilizer application saves money and protects the environment. Excess fertilizer leaches into the air, soil and water and pollutes lakes, rivers and aquifers. Nitrogen fertilizers can create massive algae blooms in the ocean that are fed by excess nutrient runoff. When algae populations get too large, they die and their natural decomposition depletes the water of oxygen leading to fish suffocation and death.
Farmers try to apply the optimum amount of fertilizer to get maximum possible growth from plants. Too much fertilizer leads to waste; however, too little fertilizer lowers crop yield unnecessarily. A basic strategy for finding the optimum fertilizer application rate begins with applying an excess of fertilizer to a small section of a field. In this “N-rich strip” plants are not limited by lack of fertilizer. During the growing season the progress of plants in the field is compared to that of plants in the N-rich strip. The difference in growth rates is used to estimate how much fertilizer to put on the field.
Plant growth rate is estimated by using optical sensors to measure Normalized Difference Vegetative Index (NDVI). NDVI is based on measurements of plant reflectivity at different wavelengths:
      N    ⁢                  ⁢    D    ⁢                  ⁢    V    ⁢                  ⁢    I    =                    r        NIR            -              r        VIS                            r        NIR            +              r        VIS            where rNIR is infrared (e.g. 780 nm) reflectivity and rVIS red (e.g. 660 nm) reflectivity. Vigorous plants absorb red and reflect infrared, leading to high NDVI readings.
NDVI can be measured on-the-fly by arrays of sensors mounted to a fertilizer spray truck. Combined with variable rate sprayers, this leads to fertilizer application rates customized over areas as small as a few square meters. Such systems are routinely used on large farms in agriculturally advanced countries. These systems are expensive, however, and have not yet been introduced in less agriculturally developed regions and are also not affordable for small research projects.
What are needed are easier, cheaper systems for estimating optimum fertilizer rates. These systems should serve the needs of farmers who are just beginning to adopt precision farming techniques. These systems should also be appropriate, from performance and cost points of view, for training and research purposes.